Cross reference is made to the following co-pending patent applications, each being assigned to the same assignee as the present invention and the teachings included herein by reference:
The present invention generally relates to an optical communication link, and more particularly, to a system and method for identifying which optical carriers being transmitted through an optical fiber are contributing to the generation of an undesired mixing product.
Optical communication networks serve to transport information at high data rates between a number of physical sites, commonly referred to as nodes. Each of these nodes are interconnected with the various other nodes by information conduits, commonly referred to as links. These links are comprised of at least one, and usually several, optical fibers. Information is usually presented to the optical communication network in the form of time-domain electrical data signals, and may represent any combination of telephony, video, or computer data in a variety of formats. The electrical data signals are processed to intensity modulate the light output of respective laser diodes of an optical transmitter to generate modulated optical carriers suitable for traveling through optical fibers.
In order to increase the utilization of the optical communications fiber, wavelength division multiplexing (WDM) is typically employed to send multiple optical carriers along the optical fiber, each optical carrier having a different wavelength. Design engineers strive to maximize capacity of the communications network by transmitting as many optical carriers as possible over the optical fiber. While two-wavelength and four-wavelength optical systems are fairly common, the telecom industry is already planning for ways to crowd eight and sixteen optical carriers or channels within the so-called erbium band, roughly covering the range of wavelengths from 1530 to 1560 nanometers. The channels are usually evenly spaced to make best use of the band, and to allow use of comb-generating implements as references for tuning the carriers.
Unfortunately, evenly spaced carriers can interfere with one another and create mixing products if the optical power levels are high enough to cause the fiber medium to exhibit a non-linear refractive index. Mixing products are generated by several optical carriers of different wavelengths beating together and generating unwanted spurious wavelengths that can interfere with one or more of the desired optical carriers. The most common phenomenon that creates unwanted mixing products is referred to as four-wave mixing in which the difference of two wavelengths is summed with a third wavelength, yielding a fourth wavelength as a mixing product. In an exactly evenly spaced carrier plan, the resulting mixing product will coincide with one of the desired carrier wavelengths.
In commonly assigned U.S. Pat. No. 5,600,467, there is disclosed a technique for taking an inventory of observed unwanted byproducts, calculating the shifts in carrier wavelengths needed to move the unwanted wavelengths away from carrier wavelengths, and then fine tuning the carriers to accomplish the improved shifts. A pre-established look-up table of known interactions among the wavelengths is utilized to determine the shifting of one or several carrier wavelengths to reduce or move the unwanted mixing products.
In some cases, it is rather difficult to determine which optical carriers are contributing to the generation of a particular mixing product since several possible combinations of multiple optical carriers may produce the same spurious wavelength. Therefore, it is often not known which optical carriers must be perturbed in order to minimize or eliminate the interference. The optical carriers are typically revenue bearing data signals that can""t be interrupted to identify the source of interference.
There is desired an improved optical communication link and technique that provides a more dynamic assessment of which transmit optical carrier wavelengths are actually involved in the production of a given observed spurious wavelength in the optical fiber.
The present invention achieves technical advantages as a system and method of modulating the optical carriers with a low-level subcarrier to identify which of the optical carriers are contributing to a particular unwanted mixing product. The present invention takes advantage of the fact that if a given optical carrier is participating in the generation of a mixing product, then the subcarrier modulating the optical carrier will also appear in the mixing product, thus identifying the associated optical carrier as a participant in the generation of the particular mixing product. The present invention selectively tags the optical carriers in a variety of ways, and then analyzes the mixing product for the presence of the tags. Various embodiments of the present invention are disclosed.
According to a first preferred embodiment of the present invention, a controller selectively switches a subcarrier signal onto each of the optical carriers, one at a time, and then detects the presence or absence of the subcarrier in a spurious wavelength under observation. Since it is known which optical carrier is being modulated with the subcarrier when the subcarrier is detected in the unwanted mixing product, the optical carriers contributing to the mixing product can be identified. The first embodiment comprises an optical communication link comprising an optical fiber having a transmit end and a receive end. An optical transmitter is coupled to the optical fiber transmit end and generates a plurality of optical carriers. A modulator is coupled to the optical transmitter for selectively modulating each of the optical carriers with a subcarrier. A sensor is coupled to the optical fiber link and senses the mixing product generated by some of the plurality of optical carriers. An analyzer is coupled to the sensor for analyzing the sensed mixing product. A controller is coupled to and controls both the modulator and analyzer, whereby the controller controls the analysis of the sensed mixing product as a function of which optical carrier is being modulated with the subcarrier. The analyzer senses the presence, or absence, of the subcarrier in the mixing product as a function of which optical carrier is being modulated with the subcarrier to identify the optical carriers contributing to the mixing product. The controller may also control the amplitude or frequency of the contributing optical carriers as a function of the presence of the subcarrier in the mixing product. Preferably, the analyzer comprises a spectrun analyzer, whereby the sensor may comprise an optical selective level meter and a detector.
According to a second embodiment of the present invention, a plurality of modulators are implemented, wherein each modulator modulates a different optical carrier with a separate subcarrier. Preferably, each of the subcarriers has a different characteristic frequency. A sensor is coupled to the optical fiber link and senses the mixing product generated by some of the plurality of optical carriers. An analyzer is coupled to the sensor and analyzes the mixing product to determine the presence of the subcarriers in the mixing product. The identification of the subcarriers identifies which corresponding optical carriers are contributing to the unwanted mixing product.
In a third embodiment, each of these separate subcarriers is modulated with data identifying the optical carrier being modulated by the respective subcarrier. The tagged subcarriers identify the associated optical carriers.
According to a fourth embodiment, a subcarrier generator is utilized to selectively modulate each of the optical carriers with a subcarrier according to a predetermined pattern. Thus, each optical carrier can be modulated with a common subcarrier. The optical communication link in this embodiment comprises an optical fiber link extending between a transmit end and a receive end. An optical transmitter is coupled to the optical fiber transmit end and generates a plurality of optical carriers. The subcarrier generator is coupled to the optical transmitter and modulates the optical carriers with a subcarrier according to a pre-determined pattern. A sensor is coupled to the optical fiber link and senses a mixing product generated by some of the plurality of optical carriers. An analyzer is coupled to the sensor and analyzes the mixing product, determining the presence of one or more subcarriers at a given time in the mixing product. Since the optical carriers are modulated with the subcarrier according to the known pre-determined pattern, the analyzer determines which of the optical carriers is being modulated with the subcarrier when the subcarrier is identified as being present in the mixing product. The subcarrier may further be modulated with data to identify which optical carrier is currently being modulated. This data can be ascertained by the analyzer to further help in ascertaining the identity of the particular optical carrier or carriers contributing towards the particular mixing product.
According to a method of the present invention, the method comprises detecting which of a plurality of optical carriers transmitted in an optical fiber are contributing toward the generation of a mixing product in the optical fiber. The method comprises the steps of modulating at least one, and preferably each in turn, of the optical carriers with a subcarrier, and then analyzing the mixing product to determine the presence of the subcarrier in the mixing product to determine which optical carriers are contributing toward the generation of the mixing product. In one embodiment, the analysis of the mixing product may be controlled as a function of which optical carrier is being modulated. In another embodiment, the method comprises the step of coordinating the modulation of each of the optical carriers with the subcarrier with the analysis of the mixing product, such as by using a common controller. According to a further embodiment, the method comprises the step of modulating the optical carriers with the subcarrier according to a pre-determined pattern. In another embodiment, each optical carrier is simultaneously modulated with a separate subcarrier, each subcarrier having a different frequency from the other. Each subcarrier may be further modulated with data identifying which optical carrier is being modulated by the respective subcarrier. According to the various methods of the present invention, the amplitude or frequency of one or several optical carriers may be adjusted as a function of the analyzed mixing product to reduce or eliminate the presence of the mixing product.